/**
 * (C) Copyright IBM Corp. 2010, 2015
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 */

package com.ibm.bi.dml.lops.compile;

import java.util.Comparator;

import com.ibm.bi.dml.lops.Lop;


/**
 * 
 * Comparator class used in sorting the LopDAG in topological order. Refer to
 * doTopologicalSort_strict_order() in dml/lops/compile/Dag.java
 * 
 * Topological sort guarantees the following:
 * 
 * 1) All lops with level i appear before any lop with level greater than i
 * (source nodes are at level 0)
 * 
 * 2) Within a given level, nodes are ordered by their ID i.e., by the other in
 * which they are created
 * 
 * compare() method is designed to respect the above two requirements.
 *  
 * @param <N>
 */
public class LopComparator<N extends Lop>
		implements Comparator<N> 
{
	
	@Override
	public int compare(N o1, N o2) {
		if (o1.getLevel() < o2.getLevel())
			return -1; // o1 is less than o2
		else if (o1.getLevel() > o2.getLevel())
			return 1; // o1 is greater than o2
		else {
			if (o1.getID() < o2.getID())
				return -1; // o1 is less than o2
			else if (o1.getID() > o2.getID())
				return 1; // o1 is greater than o2
			else
				throw new RuntimeException("Unexpected error: ID's of two lops are same.");
		}
	}

}
